When single-walled carbon nanotubes (SWNTs) are
functionalized, they are mostly bundled. To exploit their
properties, they must be unbundled into individual nanotubes,
but this is difficult because of the high intermolecular
cohesive forces. A few processes yield individual tubes and
small bundles, but only in small amounts over long reaction
times using harsh solvents or costly equipment.
Researchers at Rice University led by James M. Tour have
devised a rapid and environmentally benign ‘green’ process
using ionic liquids (ILs) to exfoliate the bundles and diazonium
chemistry to derivatize predominantly individual SWNTs [Price
et al., J. Am. Chem. Soc. (2005) 127 (42), 14867].
ILs are salts that are nonvolatile liquids at room temperature
(with boiling points >200°C), and hence can be recycled. The
reaction occurs in minutes at room temperature by just
grinding the SWNTs in a mortar and pestle with an
imidazolium-based IL and a K2CO3 base. The IL’s counterion
exfoliates the negatively charged bundles by intercalating
between them. This allows the diazonium species to react
with the sidewalls of the SWNTs, appending functionalized
aryl groups and inhibiting rebundling.
Future work will include increasing the scale of the reaction
and optimizing the recycling of the ILs, says Tour.